Primary electrochemical cells are utilized for a wide variety of applications and are commonly available in a large assortment of sizes and shapes. One well known form of an electrochemical cell is a cylindrical cell. Such a cell is disclosed in U.S. Pat. No. 4,060,668 in the name of Franz Goebel and assigned to GTE Laboratories Incorporated. The cylindrical cell as disclosed in the above-mentioned patent includes a lithium anode, a carbon current collector cathode structure, and a separator interposed between the lithium anode and the carbon current collector cathode structure. The lithium anode takes the form of a cylindrical sheet physically pressed against an interior wall of a metal housing of the cell, and is arranged concentrically within the housing with the carbon current collector cathode structure and the separator. The assembly of the lithium anode, carbon current collector cathode structure and separator as described hereinabove is exposed to an electrolytic solution including a reducible soluble cathode and an electrolyte solute dissolved in the reducible soluble cathode. Suitable materials for the reducible soluble cathode and the electrolyte solute are thionyl chloride and lithium tetrachloroaluminate, respectively.
In the normal discharge of a cell such as described hereinabove, the reducible soluble cathode, such as the thionyl chloride, is catalytically reduced at the surface of the carbon current collector cathode structure. This catalytic reduction results, inter alia, in the formation of a variety of reaction products within the cell and a physical and volumetric depletion of the thionyl chloride and lithium. The life span of the cell is to a large degree dictated by the amount of lithium and thionyl chloride initially present in the cell and the rate at which the thionyl chloride and lithium are depleted by electrochemical action within the cell.
As the above-mentioned catalytic reduction of the thionyl chloride takes place at the surface of the carbon current collector cathode structure, a further, and undesirable, reduction of the thionyl chloride also takes place at those metallic portions and surfaces of the cell in physical contact with the lithium anode, that is, at the potential of the lithium anode, but not covered by the lithium anode. These portions and surfaces may include, by way of example, the bottom of the metal (e.g., stainless steel) housing of the cell and metal portions of the housing above the lithium anode. This latter reduction of the thionyl chloride, which may take place prior to normal discharge of the cell, that is, during storage of the cell, is a parasitic self-discharge reaction and can lead to an undesirable capacity loss and a premature termination of the rated life span of the cell. The rate of this self-discharge is directly proportional to the metallic area of the cell at the potential of the lithium anode, but not covered by the lithium of the anode. It is highly advantageous and desirable, therefore, that those areas of the cell at the potential of the lithium anode and at which the above-mentioned self-discharge catalytic reduction of the thionyl chloride takes place be kept to a minimum so as not to unnecessarily prematurely shorten the life span of the cell.